The present invention relates generally to closely coupled gas atomization. More particularly, it relates to methods and means by which closely coupled gas atomization processing of high melting reactive molten metal can be started and carried out with significantly reduced melt superheat.
The technology of close coupled or closely coupled atomization is a relatively new technology. Methods and apparatus for the practice of close coupled atomization are set forth in commonly owned U.S. Pat. Nos. 4,631,013; 4,801,412; and 4,619,597, the texts of which are incorporated herein by reference. As pointed out in these patents, the idea of close coupling is to create a close spatial relationship between a point at which a melt stream emerges from a melt orifice into an atomization zone and a point at which a gas stream emerges from a gas orifice to impact the melt stream as it emerges from the melt orifice into the atomization zone. Close coupled atomization is accordingly distinguished from the more familiar and conventional remotely coupled atomization by the larger spatial separation between the respective nozzles and point of impact in the remotely coupled apparatus. A number of independently owned prior art patents deal with close proximity of melt and gas streams and include U.S. Pat. Nos. 3,817,503; 4,619,845; 3,988,084; and 4,575,325.
In the more conventional remotely coupled atomization, a stream of melt may be in free fall through several inches before it is impacted by a gas stream directed at the melt from an orifice which is also spaced several inches away from the point of impact.
The remotely coupled apparatus is also characterized by a larger spatial separation of a melt orifice from a gas orifice of the atomization apparatus. Most of the prior art of the atomization technology concerns remotely coupled apparatus and practices. One reason for this is that attempts to operate closely coupled atomization apparatus resulted in many failures due to the many problems which are encountered. This is particularly true for efforts to atomize reactive metals which melt at relatively high temperatures of over 1000.degree. C. or more. The technology disclosed by the above referenced commonly owned patents is, in fact, one of the first successful closely coupled atomization practices that has been developed.
The problem of closely coupled atomization of highly reactive high temperature (above 1,000.degree. C.) metals is entirely different from the problems of closely coupled atomization of low melting metals such as lead, zinc, or aluminum. The difference is mainly in the degree of reactivity of high reacting alloys with the materials of the atomization apparatus.
One of the features of the closely coupled atomization technology, particularly as applied to high melting alloys such as iron, cobalt, and nickel base superalloys is that such alloys benefit from having a number of the additive elements in solid solution in the alloy rather than precipitated out in the alloy and the closely coupled atomization can result in a larger fraction of additive elements remaining in solid solution. For example, if a strengthening component such as titanium, tantalum, aluminum, or niobium imparts desirable sets of properties to an alloy, this result is achieved largely from the portion of the strengthening additive which remains in solution in the alloy in the solid state. In other words, it is desirable to have certain additive elements such as strengthening elements remain in solid solution in the alloy rather than in precipitated form. Closely coupled atomization is more effective than remotely coupled atomization in producing the small powder sizes which will retain the additive elements in solid solution.
Where still higher concentrations of additive elements are employed above the solubility limits of the additives, the closely coupled atomization technology can result in nucleation of precipitates incorporating such additives. However, because of the limited time for growth of such nucleated precipitates, the precipitate remains small in size and finely dispersed. It is well-known in the metallurgical arts that finely dispersed precipitates are advantageous in that they impart advantageous property improvements to their host alloy when compared, for example, to coarse precipitates which are formed during slow cooling of large particles. Thus, the atomization of such a superalloy can cause a higher concentration of additive elements, such as strengthening elements, to remain in solution, or precipitate as very fine precipitate particles, because of the very rapid solidification of the melt in the closely coupled atomization process. This is particularly true for the finer particles of the powder formed from the atomization.
In this regard, it is known that the rate of cooling of a molten particle of relatively small size in a convective environment such as a flowing fluid or body of fluid material is determined by the properties of the droplet and of the cooling fluid. For a given atomization environment, that is one in which the gas, alloy, and operating conditions are fixed, the complex function relating all the properties can be reduced to the simple proportionality involving particle size shown below, ##EQU1## where: T.sub.p =cooling rate, and
D.sub.p =droplet diameter, PA1 .sigma. and D are the droplet surface tension and diameter.
Simply put, the cooling rate for a hot droplet in a fixed atomization environment is inversely proportional to the diameter squared. Accordingly, the most important way to increase the cooling rate of liquid droplets is to decrease the size of the droplets. This is the function of effective gas atomization.
Thus it follows that if the average size of the diameter of a droplet of a composition is reduced in half, then the rate of cooling is increased by a factor of about 4. If the average diameter is reduced in half again, the overall cooling rate is increased 16 fold.
Since high cooling rates are predominantly produced by reducing droplet size, it is critical to effectively atomize the melt.
The Weber number, We, is the term assigned to the relationship governing droplet breakup in a high velocity gas stream. The Weber number may be calculated from the following expression: ##EQU2## where .rho. and V are the gas density and velocity, and
When the We number exceeds ten, the melt is unstable and will breakup into smaller droplets. The dominant term in this expression is gas velocity and thus in any atomization process it is essential to have high gas velocities. As described in the commonly owned U.S. Pat. No. 4,631,013 the benefit of close coupling is that it maximizes the available gas velocity in the region where the melt stream is atomized. In other words, the close coupling is itself beneficial to effective atomization because there is essentially no loss of gas velocity before the gas stream from the nozzle impacts the melt stream and starts to atomize it.
Because of this relationship of the particle size to the cooling rate, the best chance of keeping a higher concentration of additive elements of an alloy, such as the strengthening additives, in solid solution in the alloy is to atomize the alloy to very small particles. Also, the microstructure of such finer particles is different from that of larger particles and often preferable to that of larger particles.
For an atomization processing apparatus, accordingly the higher the percentage of the finer particles which are produced the better the properties of the articles formed from such powder by conventional powder metallurgical techniques. For these reasons, there is strong economic incentive to produce finer particles through atomization processing.
As pointed out in the commonly owned prior art patents above, the closely coupled atomization technique results in the production of powders from metals having high melting points with higher concentration of fine powder. For example, it was pointed out therein that by the remotely coupled technology only 3% of powder produced industrially is smaller than 10 microns and the cost of such powder is accordingly very high. Fine powders of less than 37 microns in diameter of certain metals are used in low pressure plasma spray applications. In preparing such powders by remotely coupled techniques, as much as 60-75% of the powder must be scrapped because it is oversized. This need to selectively separate out only the finer powder and to scrap the oversized powder increases the cost of useable powder.
Further, the production of fine powder is influenced by the surface tension of the melt from which the fine powder is produced. For melts of high surface tension, production of fine powder is more difficult and consumes more gas and energy. The remotely coupled industrial processes for atomizing such powder have yields of less than 37 microns average diameter from molten metals having high surface tensions of the order of 25 weight % to 40 weight %. A major cost component of fine powders prepared by atomization and useful in industrial applications is the cost of the gas used in the atomization. Using remotely coupled technology, the cost of the gas increases as the percentage of fine powder sought from an atomized processing is increased. Also, as finer and finer powders are sought, the quantity of gas per unit of mass of powder produced by conventional remotely coupled processing increases. The gas consumed in producing powder, particularly the inert gas such as argon, is expensive.
As is explained more fully in the commonly owned patents referred to above, the use of the closely coupled atomization technology of those patents results in the formation of higher concentrations of finer particles than are available through the use of remotely coupled atomization techniques. The texts of the commonly owned patents are incorporated herein by reference.
As is pointed out more fully in the commonly owned U.S. Pat. No. 4,631,013, a number of different methods have been employed in attempts to produce fine powder. These methods have included rotating electrode process, vacuum atomization, rapid solidification rate process and other methods. The various methods of atomizing liquid melts and the effectiveness of the methods is discussed in a review article by A. Lawly, entitled "Atomization of Specialty Alloy Powders", which article appeared in the Jan. 19, 1981 issue of the Journal of Metals. It was made evident from this article and has been evident from other sources that gas atomization of molten metals produces the finest powder on an industrial scale and at the lowest cost.
It is further pointed out in the commonly owned U.S. Pat. No. 4,631,013 that the close coupled processing as described in the commonly owned patents produces finer powder by gas atomization than prior art remotely coupled processing.
A critical factor in the close coupled gas atomization processing of molten metals is the melting temperature of the molten metal to be processed. Metals which can be melted at temperatures of less than 1000.degree. C. are easier to atomize than metals which melt at 1500.degree. to 2000.degree. C. or higher, largely because of the degree of reactivity of the metal with the atomizing apparatus at the higher temperatures. The nature of the problems associated with close coupled atomization is described in a book entitled "The Production of Metal Powders by Atomization", authored by John Keith Beddow, and printed by Haden Publishers, as is discussed more fully in the the commonly owned U.S. Pat. No. 4,631,013.
The problems of attack of liquid metals on the atomizing apparatus is particularly acute when the more reactive liquid metals or more reactive constituent of higher melting alloys are involved. The more reactive metals include titanium, niobium, aluminum, tantalum, and others. Where such ingredients are present in high melting alloys such as the superalloys, the tendency of these metals to attack the atomizing apparatus itself is substantial. For this reason, it is desirable to atomize a melt at as low a temperature as is feasible.
What has been discovered regarding the conventional close coupled atomization is that there is a tendency for the atomization apparatus to freeze up and this freeze-up problem becomes more acute as the degree of superheat in the molten metal to be atomized is reduced.